Sorry I can't help with references. Have you checked
SKYbrary Aviation Safety? They'd be my preferred starting point for research.
If I had to guess, I'd say runway incursions are more likely to happen during taxi-out crossings. Like rayfill, I first thought fatigue would be a major contributing factor, so taxi-in crossings seemed more critical, but then again the workload in the taxi-out phase is higher due to briefings, cabin announcements, co-ordination with ATC etc., which suggests taxi-out crossings. As I say, I'm guessing.